CHEVY 1500 HYDRAULIC CLUTCH

1990 Chevrolet Truck • 6 cylinder 4WD Manual • 185,000 miles

When I push in my clutch pedal, on 1990 GMC 1500, the clutch WILL disengage but then 'looses pressure' and clutch engages (while pedal is still on floor, in gear, tries to 'go'). Is this a symptom of the slave cylinder needing to bled or other?